Stonehenge is hiding a secret: the source of one of its stones.

Researchers know that the prehistoric circle's outer stones come from nearby areas in England. But even after a century of study, the Altar Stone near the center of Stonehenge remains difficult to understand.

The Altar Stone belongs to a group of Stonehenge building blocks known as the bluestones, which came from areas far from Stonehenge. Researchers have found that some of the bluestones come from as far as 225 kilometers away.

"It's a massive job of transport," says Richard Bevins, an earth scientist at Aberystwyth University in Wales. His team is looking for the source of the Altar Stone. Uncovering the stone's origins could suggest which ancient groups of people contributed to the building.

Scientists have been working to find the rock's origins since

1923. A report about the Altar Stone's minerals back then suggested it might have come from a set of rocks in Wales near where other bluestones cane from. Bevins' team decided to revisit the rock's riddle with modern techniques.

In 2021, the team analyzed (分析) the Altar Stone's chemical makeup using X-rays. The X-ray method showed that the Altar Stone has high levels of the element barium (钡). But the stone's makeup didn't seem to match the rocks in Wales.

In the new study, the team collected 58 samples (样本) from a wider area in England and Wales. Of the 58 sample stones, four had high barium levels similar to the Altar Stone. The team then compared the overall mineral makeup of those four stones with the Altar Stone. But none were a match.

"Maybe we've been looking in the wrong area, and maybe we've possibly been looking at rocks of the wrong age," Bevins says, "It's not clear exactly how old the Altar Stone is." So scientists may need to consider stone sources that are younger than the ones they have looked at so far.